The invention relates to a drive system for a ball mill and a method for operating a ball mill.
A ball mill is a device for coarse, fine and ultrafine reduction or homogenization of grinding stock. It comprises a grinding chamber, normally formed by an inner chamber of a drum which can be made to rotate and in which the grinding stock is reduced by means of a grinding medium. Ball mills are used for both wet grinding and dry grinding. In particular, the term ball mill includes rolling mills, drum mills and tube mills. Grinding mediums used in ball mills include not only spherical grinding mediums but also other shapes such as cylindrical grinding mediums, etc.
Ball mills are used for grinding ore in mining operations, for example. In other branches of industry, substances to be ground may include ceramic materials or material mixtures. Ball mills are used in the production of slips in the ceramics industry, for example.
Various drive designs exist for such mills. The use of gearless direct drives in the form of ring motors is known. The operation of a ball mill by means of a slipring motor is simpler and less costly.
When a ball mill is stationary, the material to be ground (though unconsolidated itself) can solidify in the mill. This occurs, for example, if the mill is switched off for some hours or days for the purpose of maintenance. The grinding stock then solidifies and adheres to the inner wall of the drum. Such a situation is referred to as “caking” or “frozen charge”. If the drum of the ball mill is made to rotate again after such caking has occurred, nearly all of the material to be ground, including the added grinding medium, remains adhered to the mill wall as a unit in a solid state and is rotated as such from an approximately horizontal position. If the drum of the mill then reaches a critical angle of rotation and the caking therefore reaches a critical inclined position, it is usual for a large part of the caking to suddenly become detached. Due to the considerable mass of the caking that falls, a ball mill can be seriously damaged or even ruined.
In the case of smaller mills, which are normally driven by slipring motors, automatic caking recognition is known, e.g. in the form of a “Mill Safety Start System” from SDG Technologies. However, the caking cannot be dispersed using the known system. In order to disperse such caking, it is usual to add water to the caking and then disperse it, possibly using mechanical means.
The known “Mill Safety Start System” therefore merely allows the “frozen charge” state, i.e. the presence of caking, to be captured or recognized. Switching off the mill likewise takes place automatically. However, the caking must still be detached manually, e.g. by flooding or mechanical means. The mill is then restarted, likewise manually.
DE 10 2004 015 057 A1 discloses a method, particularly for large mills comprising a ring motor, for decompacting caking by means of selectively actuating the motor of the mill. The known method is easy to implement in a mill which features a ring motor, as it consists in merely programming the motor regulator. Unlike slipring motors, a corresponding regulator is already present. Such a method is used in the “Frozen Charge Shaker” product from Siemens, for example. In this context, said method essentially consists in merely programming the regulator of the ring motor, since a corresponding regulator is already present in the system. The system which is known for ring motors cannot be applied in the case of slipring motors for the reason that the drive in the form of a slipring motor is merely controlled, but is not regulated. A forward and backward movement or shaking of the caking in the mill as applied in the known method is therefore not possible using standard drives, i.e. slipring motors.